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Bacteria must coordinate their growth rate, shape, and division to survive and flourish, yet how these cellular properties are maintained in the face of environmental stresses is poorly understood. Working with Escherichia coli , we show that activating the Rcs phosphorelay, an envelope stress-signaling system, in the absence of external stresses slows growth, shortens cells, and increases the concentration of the key division protein FtsZ, leading to more closely spaced division sites. Depleting the levels of IgaA, a regulator of the Rcs pathway, yielded similar phenotypes. However, activating Rcs via drug-induced cell-wall disruption did not affect growth rate, indicating that the physiological impact of this pathway depends on the context of activation. Our findings reveal links among cell growth, shape homeostasis, and cell envelope stress. Understanding this coupling further will provide new avenues to predict and modulate bacterial growth and physiology during stress.

M. tuberculosis remains an important public health threat, with more than one million deaths every year. The pathogen’s ability to survive in the human host for decades highlights the importance of understanding how this bacterium regulates and coordinates its metabolism, cell envelope elongation, and growth. The IMD is a membrane structure that associates with the subpolar growth zone of actively growing mycobacterial cell, but its existence is only known in a non-pathogenic model, M. smegmatis . Here, we demonstrated the presence of the IMD in M. tuberculosis , making the IMD an evolutionarily conserved plasma membrane compartment in mycobacteria. Furthermore, our study revealed that the IMD may function as the factory for synthesizing phenolic glycolipids, virulence factors produced by slow-growing pathogenic species.

Current data from bacterial pathogens of animals and from bacterial symbionts of plants support some of the more general proposed functions for lipopolysaccharides (LPS) and underline the importance of LPS structural versatility and adaptability. Most of the structural heterogeneity of LPS molecules is found in the O-antigen polysaccharide. In this review, the role and mechanisms of this striking flexibility in molecular structure of the O-antigen in bacterial pathogens and symbionts are illustrated by some recent findings. The variation in O-antigen that gives rise to an enormous structural diversity of O-antigens lies in the sugar composition and the linkages between monosaccharides. The chemical composition and structure of the O-antigen is strain-specific (interstrain LPS heterogeneity) but can also vary within one bacterial strain (intrastrain LPS heterogeneity). Both LPS heterogeneities can be achieved through variations at different levels. First of all, O-polysaccharides can be modified non-stoichiometrically with sugar moieties, such as glucosyl and fucosyl residues. The addition of non-carbohydrate substituents, i.e. acetyl or methyl groups, to the O-antigen can also occur with regularity, but in most cases these modifications are again non-stoichiometric. Understanding LPS structural variation in bacterial pathogens is important because several studies have indicated that the composition or size of the O-antigen might be a reliable indicator of virulence potential and that these important features often differ within the same bacterial strain. In general, O-antigen modifications seem to play an important role at several (at least two) stages of the infection process, including the colonization (adherence) step and the ability to bypass or overcome host defense mechanisms. There are many reports of modifications of O-antigen in bacterial pathogens, resulting either from altered gene expression, from lysogenic conversion or from lateral gene transfer followed by recombination. In most cases, the mechanisms underlying these changes have not been resolved. However, in recent studies some progress in understanding has been made. Changes in O-antigen structure mediated by lateral gene transfer, O-antigen conversion and phase variation, including fucosylation, glucosylation, acetylation and changes in O-antigen size, will be discussed. In addition to the observed LPS heterogeneity in bacterial pathogens, the structure of LPS is also altered in bacterial symbionts in response to signals from the plant during symbiosis. It appears to be part of a molecular communication between bacterium and host plant. Experiments ex planta suggest that the bacterium in the rhizosphere prepares its LPS for its roles in symbiosis by refining the LPS structure in response to seed and root compounds and the lower pH at the root surface. Moreover, modifications in LPS induced by conditions associated with infection are another indication that specific structures are important. Also during the differentiation from bacterium to bacteroid, the LPS of Rhizobium undergoes changes in the composition of the O-antigen, presumably in response to the change of environment. Recent findings suggest that, during symbiotic bacteroid development, reduced oxygen tension induces structural modifications in LPS that cause a switch from predominantly hydrophilic to predominantly hydrophobic molecular forms. However, the genetic mechanisms by which the LPS epitope changes are regulated remain unclear. Finally, the possible roles of O-antigen variations in symbiosis will be discussed.

The gram-negative cell envelope is a barrier that protects the cell from environmental stress. Therefore, the synthesis of each layer of this envelope needs to be closely coordinated throughout growth and division. Here, we investigated SanA, a protein in Escherichia coli K-12 that affects envelope permeability under cellular stress, including nutrient limitation and high temperature. We found that SanA plays a key role in maintaining the permeability barrier when precursor levels for peptidoglycan (PG) synthesis are elevated, linking envelope integrity to balanced septal PG production during cell division. Our results suggest that SanA modulates substrate availability to preserve envelope function, and that in its absence, imbalanced substrate flux to septal PG synthesis disrupts septum formation and compromises barrier integrity.

DD-peptidases, including carboxypeptidases and endopeptidases, are crucial for maintaining cell envelope homeostasis, with distinct roles for each enzyme in cell wall biogenesis and structural integrity. The enzymatic characterization presented in this study not only advances our understanding of fundamental Acinetobacter baumannii biology but also highlights these enzymatic activities as targets for the development of innovative therapeutic strategies to combat infections caused by this multidrug-resistant microbe.

Pseudomonas aeruginosa is an opportunistic bacterial pathogen that causes chronic infection in humans by forming protective, multicellular structures called biofilms. The strong natural resistance of bacterial biofilms to antibiotic and immune clearance presents a major therapeutic obstacle in P. aeruginosa disease management. As these structures often assemble on surfaces, i.e. host tissues or indwelling medical devices, the ability of P. aeruginosa to sense and respond to surface contact is a key step in initiating biofilm formation. We report that the Cpx signaling system in P. aeruginosa is activated upon surface attachment and operates independently of other known surface-sensing systems. Cpx responds to cellular stress, particularly disruptions to cell-surface proteins, suggesting that stress generated by bacterial surface adhesion is a relevant biofilm-inducing signal. These findings expand knowledge of surface-sensing mechanisms in P. aeruginosa and link surface recognition to a variety of other disease-related cellular processes regulated by the Cpx system.

Tuberculosis, caused by Mycobacterium tuberculosis (Mtb), remains the world’s deadliest bacterial infection, in part because the bacterium’s unique cell envelope makes it highly resistant to antibiotics. Understanding how this protective barrier is built is essential for developing better treatments. In this study, we discovered that two previously uncharacterized lipoproteins help maintain the integrity of the mycobacterial cell envelope and contribute to drug resistance. Surprisingly, instead of acting as transport proteins as expected by structural similarity, these molecules regulate enzymes that assemble the bacterial envelope. This discovery highlights a previously unrecognized layer of control in envelope construction and opens new directions for targeting Mtb’s defenses with future therapies.

Monoderm bacteria possess a cell envelope made of a cytoplasmic membrane and a cell wall, whereas diderm bacteria have and extra lipid layer, the outer membrane, covering the cell wall. Both cell types can also produce extracellular protective coats composed of polymeric substances like, for example, polysaccharidic capsules. Many of these structures form a tight physical barrier impenetrable by phage virus particles. Tailed phages evolved strategies/functions to overcome the different layers of the bacterial cell envelope, first to deliver the genetic material to the host cell cytoplasm for virus multiplication, and then to release the virion offspring at the end of the reproductive cycle. There is however a major difference between these two crucial steps of the phage infection cycle: virus entry cannot compromise cell viability, whereas effective virion progeny release requires host cell lysis. Here we present an overview of the viral structures, key protein players and mechanisms underlying phage DNA entry to bacteria, and then escape of the newly-formed virus particles from infected hosts. Understanding the biological context and mode of action of the phage-derived enzymes that compromise the bacterial cell envelope may provide valuable information for their application as antimicrobials.

In double-membraned bacteria, non-equilibrium processes that occur at the outer membrane are typically coupled to the chemiosmotically energized inner membrane. TolA and TonB are homologous proteins which energetically couple inner membrane motor proteins to the essential processes of outer membrane stabilization and substrate import, respectively. The evolutionary trajectories of these proteins have been difficult to elucidate due to low-sequence conservation, yet they are thought to transduce force similarly. Here, this problem was addressed using structural prediction approaches to identify and annotate force transduction operons to trace their distribution and evolutionary origins. In the process, we identify a novel outer membrane-tethering system and a previously unknown family of monomeric force transducers. This approach revealed putative tolA genes, and thus the core organizational principles of the tol-pal operon throughout diverse bacterial taxa. We discovered that the α-helical structure of the periplasm-spanning domain II of TolA previously thought its hallmark, is anomalous amongst most Tol-Pal systems. This structure is mainly prevalent in γ-proteobacteria, likely in adaptation to their lifestyle. Comparison of Tol-Pal and Ton system distribution suggests that TolA emerged from a TonB paralogue and co-emerged with Pal, the outer membrane-tethering lipoprotein that functionalizes the Tol-Pal system. We also determined that TolB, the Pal-mobilizing protein, likely emerged from a family of outer membrane proteins; and CpoB, a periplasmic factor that coordinates peptidoglycan remodeling with cell division, was originally a lipoprotein present in the ancestral Tol-Pal system. The extensive conservation of the Tol-Pal system throughout Gracilicutes highlights its significance in bacterial cell biology.

The presence of a cell membrane is one of the major structural components defining life. Recent phylogenomic analyses have supported the hypothesis that the last universal common ancestor (LUCA) was likely a diderm. Yet, the mechanisms that guided outer membrane (OM) biogenesis remain unknown. Thermotogae is an early-branching phylum with a unique OM, the toga. Here, we use cryo-electron tomography to characterize the in situ cell envelope architecture of Thermotoga maritima and show that the toga is made of extended sheaths of β-barrel trimers supporting small (∼200 nm) membrane patches. Lipidomic analyses identified the same major lipid species in the inner membrane (IM) and toga, including the rare to bacteria membrane-spanning ether-bound diabolic acids (DAs). Proteomic analyses revealed that the toga was composed of multiple SLH-domain containing Ompα and novel β-barrel proteins, and homology searches detected variable conservations of these proteins across the phylum. These results highlight that, in contrast to the SlpA/OmpM superfamily of proteins, Thermotoga possess a highly diverse bipartite OM-tethering system. We discuss the implications of our findings with respect to other early-branching phyla and propose that a toga-like intermediate may have facilitated monoderm-to-diderm cell envelope transitions.